This invention relates to hollow fiber fluid treatment devices, particularly such devices used for mass or energy exchange through the membrane, i.e. heat exchangers, dialyzers, and in particular membrane oxygenators for the oxygenation of blood in cardiopulmonary bypass blood circuits. Membrane oxygenators direct blood passing through an extracorporeal circuit into contact with a surface or membrane, usually a hollow fiber membrane, through which gas can diffuse or be transferred. The surfaces or membranes are used to transfer oxygen and carbon dioxide between the blood and an oxygen-bearing gas.
Hollow fiber fluid treatment devices have been developed for uses other than oxygenation and some are shown in the following U.S. Pat. Nos.: 3,536,611 to DeFilippi et al., 3,557,962 to Kohl, 3,794,468 to Leonard, 3,957,648 to Roget et al., 4,020,230 to Mahoney, 4,140,637 to Walter, 4,172,794 to Sigdell, and 4,368,124 to Brumfield.
Turning to fluid treatment apparatus for oxygenators, U.S. Pat. No. 4,374,802 to Fukusawa, U.S. Pat. No. 4,376,095 to Hasegawa, 4,657,743 to Kanno, and 4,698,207 to Bringham et al. generally disclose an oxygenator with lengthwise-extending hollow fibers which contain the blood; gas is passed over the fibers. U.S. Pat. Nos. 4,620,965 to Fusakawa, 4,659,549 to Hamada, and EPO published application 0.176.671 generally disclose an oxygenator with similar longitudinally extending fibers containing the oxygen-rich gas; blood flows over the fibers. Another disclosure along this line is found in U.S. Pat. No. 4,645,645 to Martinez, et al. Yoshida, U.S. Pat. No. 3,998,593 discloses a membrane oxygenator having alternate layers of rectangular polymer membranes and rectangular mesh spacers forming alternate gas channels and blood channels.
Other oxygenators have been developed disclosing helically wound fibers, for example, U.S. Pat. No. 4,306,018 to Kirkpatrick, U.S. Pat. No. 4,715,953 to Leonard, Japanese Laid Open Patent Application Ser. No. 286426/86 to Japan Medical Supply Co., Ltd.. U.S. Pat. No. 4,424,190 to Mather III, Bringham et al., mentioned above, and U.S. Pat. No. 4,808,378 to Nakanishi. An invention directed to a method of winding a bundle of hollow fibers around a core for an oxygenator is disclosed in U.S. Pat. No. 4,572,446 to Leonard.
Blood is passed through a tubular membrane containing a group of capillary membranes extending spirally through it in U.S. Pat. No. 3,893,926, to Awad, and a somewhat similar arrangement is said to be applicable to oxygenators in U.S. Pat. Nos. 3,963,622 and 4,246,120 to Baudet, et al.
A diffusion membrane unit said to be applicable to oxygenators is disclosed in U.S. Pat. No. 4,346,006 to Kopp, et al. The unit includes a plurality of separate, parallel membrane tubes formed into a flat, substantially two-dimensional array joined by adhesive in spaced relation to each other for improved exposure of the exterior of the tubular membrane to the passing fluid. The adhesive may be a material similar to the membrane material and extends continuously across the parallel membrane tubes. In another embodiment, the unit contains a plurality of planar, single-layered arrays of parallel capillary tubes defining out of phase sinuous paths.
A device for the extracorporeal treatment of blood is disclosed in U.S. Pat. No. 4,428,403 to Lee, et al. It consists of hollow tubing with plastic monofilament wrapped around its outer surface in a generally spiral configuration, preferably then formed into a coil in which the coils are spaced apart by the monofilament.
A membrane oxygenator is also disclosed in U.S. Pat. No. 4,622,206 to Torgeson. It has a housing with hollow fibers extending cross-wise, where the blood flows transversely over the fibers. The fibers are placed in a layer at a center to center spacing of about 1.5 to 4 times the diameter of the fibers. Filaments may be positioned between at least some of the fibers to improve spacing and facilitate more efficient transfer. Fibers may be wound around a frame member and adjacent layers preferably have fibers angularly offset with respect to each other.
One primary disadvantage, however, of commercially available membrane oxygenators containing membrane units such as the above pertains to the relatively large "priming volume" of the extracorporeal circuit which includes such devices. The total internal volume of the extracorporeal circuit, which includes the oxygenation and heat exchange devices as well as other devices, is flushed out before surgery to remove any extraneous gas from the extracorporeal circuit. "Priming" is typically performed with a biocompatible solution, e.g. a saline solution, which then mixes with the patient's blood, causing dilution of the blood cells, and in particular the red blood cells for a given volume of fluid. To minimize such hemodilution, donor blood may have to be introduced into the diluted blood passing through the extracorporeal circuit.
While the addition of donor blood ma)k reduce hemodilution, it presents other complications, such as compatibility problems between the donor blood and patient's blood and complications associated with blood-borne diseases. As a result, other measures can be taken to correct the effects of hemodilution, such as using hemoconcentrators to concentrate the blood cell count. However, they are expensive and cumbersome to operate, and may injure the blood cells.
It is thus apparent that it would be advantageous to design a very efficient hollow fiber membrane apparatus in order to reduce the priming volume of the oxygenator. The major impediment to doing so, however, is the requirement of providing sufficient surface area for both heat exchange and for oxygenation. As to oxygenation, this requires a well-designed hollow fiber fluid treatment apparatus within the oxygenator to obtain sufficient oxygen transfer rates.
Another problem which may occur with membrane oxygenators is injury to or deterioration of the blood as it passes through or over the hollow fiber membranes. Stagnant areas result in thrombogenicity, while rapid flow rates in some areas result in shear stresses which cause hemolysis. Thus, it is desirable to produce a hollow fiber system with as uniform and even blood flow as possible, in order to minimize blood injury.